Microlithography is used for producing microstructured components, for example integrated circuits. The microlithographic process is carried out with a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements.
The mirrors may for example be fastened to a supporting frame (force frame) and be designed as at least partially manipulable, in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the pm range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be compensated for.
Usually, lithography apparatuses are set up at the producer. In particular, this includes positioning and aligning the mirrors with respect to one another. “Positioning” means a movement of a corresponding mirror in up to three translational degrees of freedom. “Aligning” means a movement of a corresponding mirror in up to three rotational degrees of freedom. Positioning and aligning the mirrors is repeated until an image that is produced at the entry side of the projection lens of the lithography apparatus is imaged correctly at the exit side of the lens. Subsequently, the mirrors are fixed in their respective spatial orientation. Qualification of the lithography apparatus is completed herewith.
In order now to bring such a lithography apparatus to the customer, the latter—already on account of its large dimensions—are typically dismantled into its individual parts. Then, the process described above of positioning and aligning the mirrors has to be repeated at the customer. This process can be very complicated—not only because of the multiplicity of individual positioning and alignment steps, but also because of the increasing size of the individual mirrors (partly with diameters greater than 2 m) and a correspondingly large weight of the mirrors.
Therefore, the prior art has disclosed approaches for simplifying the process described above.
By way of example, JP 2004-128307 A thus describes a projection lens which includes two partial lenses. The partial lenses each contain three mirrors. An intermediate image plane is formed between the two partial lenses. Consequently, the mirrors within each partial lens can be positioned and aligned with respect to one another. The correct position and alignment of the mirrors is checked on the basis of the intermediate image plane. After qualification of a respective partial lens, the partial lenses are assembled to form the lens.
US 2011/0001945 A1 describes an approach in which a projection lens likewise is subdivided into a plurality of partial lenses. The partial lenses are positioned and aligned with respect to one another at the producer. The position and alignment is checked and stored via appropriate sensors. For the purposes of transportation to the customer, the lens is dismantled, once again, into its partial lenses. At the customer, the lens can easily be assembled again from the partial lenses with the aid of the sensors and stored data, with the original positioning and alignment of the partial lenses with respect to one another being reproduced.